This document describes porous polymer membranes including metal nanoparticles synthesized and immobilized in-situ within the pores of the membrane while simultaneously being reduced and capped with a green reducing and capping agent. Advantageously, the resulting nanoparticles are protected from oxidation and agglomeration and thereby provide more efficient and effective reductive degradation of toxic chlorinated organic compounds and other target contaminants or pollutants of a water supply.